The present invention relates to a method and apparatus for suppressing noise normally propagating outwardly from within the inlet of a jet propulsion engine, and more particularly to a method and apparatus for suppressing the noise generated by the tips of the fan blades in a turbofan jet propulsion engine.
The tips of the fan blades of a turbofan jet propulsion engine travel at high velocities. At these velocities, sonic disturbances are generated in the airstream passing through the blades. These disturbances are propagated outwardly and forwardly through the opening of the inlet as sound waves. Attempts have been made to reduce the level of the sound waves propagated out of the inlet, but have not met with great success. One suggestion of the prior art for reducing the sound level of the forwardly propagated sound waves is to reduce the fan speed sufficiently to obtain low velocities at the tips of the fan blades. This suggestion is an inadequate solution as a high speed turbine cannot effectively drive a low speed fan without interposing a reduction gear box between the turbine and the fan. Gear boxes are undesirable in jet propulsion engines as they generate a substantial amount of noise themselves and add weight to the engine as well as presenting an initial cost and a continuing maintenance problem. Another suggestion for reducing the fan blade tip noise is to permanently position an acoustic shield, a member having at least a surface layer of acoustically treated material thereon, in the inlet as well as to line the inlet with acoustically absorbent material. These suggested remedies also have drawbacks, as the permanently positioned, prior art acoustic shields detrimentally affect engine performance under all operating conditions including those conditions when not required for noise reduction. In addition, prior art acoustic shields add a substantial amount of weight to the engines.
It is therefore an object of the present invention to provide an acoustic shield that will suppress the noise generated by the fan blade tips of a turbofan jet propulsion engine and propagated outwardly from the opening of the inlet of the engine. Further objects of the present invention are to provide: a lightweight acoustic shield; an acoustic shield that can be deployed to reduce the sound level created by the fan blade tips and that can be retracted when the sound level created by the engine is not a significant problem; to provide a sound attenuating device that does not significantly affect the operating characteristics of a turbofan jet propulsion engine either when retracted or deployed; to provide a relatively lightweight, easily manufactured sound attenuating device; and to provide several means for accomplishing the prime objective of suppressing the sound emissions from a turbofan jet propulsion engine.